Process for making dialkyl carbonates

ABSTRACT

A process for the production of dialkyl carbonates from the reaction of alcohol, for example C 1 -C 3  alcohols, with urea is disclosed wherein the water and ammonium carbamates impurities in the feed are removed in a prereactor. The water is reacted with urea in the feed to produce ammonium carbamate which is decomposed along with the ammonium carbamates originally in the feed to ammonia and carbon dioxide. In addition some of the urea is reacted with the alcohol in the first reactor to produce alkyl carbamate which is a precursor to dialkyl carbonate. Dialkyl carbonates are produced in the second reaction zone. The undesired by-product N-alkyl alkyl carbamates are continuously distilled off from the second reaction zone along with ammonia, alcohol and dialkyl carbonates under the steady state reactor operation. N-alkyl alkyl carbamates can be converted to heterocyclic compounds in a third reaction zone to remove as solids from the system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the production of dialkylcarbonates, particularly C₁-C₃ dialkyl carbonates wherein the reactionoccurs simultaneously with separation of the reactants and the carbonateproducts. More particularly the invention relates to a process whereinalcohol is reacted with urea and/or alkyl carbamate in the presence of acomplex compound catalyst. More particularly the invention relates to aprocess wherein feed stream impurities are removed to produce stablecatalyst performance, improved reaction rates and trouble freedownstream operation of equipment.

2. Related Art

Dialkyl carbonates are important commercial compounds, the mostimportant of which is dimethyl carbonate (DMC). Dimethyl carbonate isused as a methylating and carbonylating agent and as a raw material formaking polycarbonates. It can also be used as a solvent to replacehalogenated solvents such as chlorobenzene. Although the current priceof both dimethyl carbonate and diethyl carbonate is prohibitivelyexpensive to use as fuel additive, both could be used as an oxygenate inreformulated gasoline and an octane component. Dimethyl carbonate has amuch higher oxygen content (53%) than MTBE (methyl tertiary butyl ether)or TAME (tertiary amyl methyl ether), and hence not nearly as much isneeded to have the same effect. It has a RON of 130 and is less volatilethan either MTBE or TAME. It has a pleasant odor and, unlike ethers, ismore readily biodegradable.

In older commercial processes dimethyl carbonate was produced frommethanol and phosgene. Because of the extreme toxicity and cost ofphosgene, there have been efforts to develop better, non-phosgene basedprocesses.

In one new commercial process, dimethyl carbonate is produced frommethanol, carbon monoxide, molecular oxygen and cuprous chloride viaoxidative carbonylation in a two-step slurry process. Such a process isdisclosed in EP 0 460 735 A2. The major shortcomings of the process arethe low production rate, high cost for the separation of products andreactants, formation of by-products, high recycle requirements and theneed for corrosion resistant reactors and process lines.

Another new process is disclosed in EP 0 742 198 A2 and EP 0 505 374 B1wherein dimethyl carbonate is produced through formation of methylnitrite instead of the cupric methoxychloride noted above. Theby-products are nitrogen oxides, carbon dioxide, methylformate, etc.Dimethyl carbonate in the product stream from the reactor is separatedby solvent extractive distillation using dimethyl oxalate as the solventto break the azeotropic mixture. Although the chemistry looks simple andthe production rate is improved, the process is very complicated becauseof the separation of a number of the materials, balancing materials invarious flow sections of the process, complicated process control anddealing with methyl nitrite, a hazardous chemical.

In another commercial process dimethyl carbonate is produced frommethanol and carbon dioxide in a two-step process. In the first stepcyclic carbonates are produced by reacting epoxides with carbon dioxideas disclosed in U.S. Pat. Nos. 4,786,741; 4,851,555 and 4,400,559. Inthe second step dimethyl carbonate is produced along with glycol byexchange reaction of cyclic carbonates with methanol. See for example Y.Okada, et al “Dimethyl Carbonate Production for Fuel Additives”, ACS,Div. Fuel Chem., Preprint, 41(3), 868, 1996, and John F. Knifton, et al,“Ethylene Glycol-Dimethyl Carbonate Cogeneration”, Journal of MolecularChemistry, vol. 67, pp 389-399, 1991. While the process has itsadvantages, the reaction rate of epoxides with carbon dioxide is slowand requires high pressure. In addition, the exchange reaction of thecyclic carbonate with methanol is limited by equilibrium and methanoland dimethyl carbonate form an azeotrope making separation difficult.

It has been known that dialkyl carbonates can be prepared by reactingprimary aliphatic alcohols such as methanol with urea (1) in thepresence of various heterogeneous and homogeneous catalysts such asdibutyltin dimethoxide, tetraphenyltin, etc. See for example P. Ball etal, “Synthesis of Carbonates and Polycarbonates by Reaction of Urea withHydroxy Compounds”, C1 Mol. Chem., vol. 1, pp 95-108, 1984. Ammonia is acoproduct and it may be recycled to urea (2) as in the followingreaction sequence.

Carbamates are produced at a lower temperature followed by production ofdialkyl carbonates at higher temperature with ammonia being produced inboth steps.

As noted the above two reactions are reversible under reactionconditions. The order of catalytic activity of organotin compounds isR₄Sn<R₃SnX<<R₂SnX₂, wherein X=Cl, RO, RCOO, RCOS. A maximum reactionrate and minimum formation of by-products are reported for dialkyl tin(IV) compounds. For most catalysts (Lewis acids), higher catalystactivity is claimed if the reaction is carried out in the presence of anappropriate cocatalyst (Lewis base). For example, the preferredcocatalyst for organic tin (IV) catalysts such as dibutyltindimethoxide, dibutyltin oxide, etc. are triphenylphosphine and4-dimethylaminopyridine. However, thermal decomposition of intermediatealkyl carbamates and urea to isocyanic acid (HNCO) or isocyanuric acid((HNCO)₃) and alcohol or ammonia (a coproduct of urea decomposition) isalso facilitated by the organotin compounds such as dibutyltindimethoxide or dibutyltin oxide employed in the synthesis of dialkylcarbamates. WO 95/17369 discloses a process for producing dialkylcarbonate such as dimethyl carbonate in two steps from alcohols andurea, utilizing the chemistry and catalysts published by P. Ball et al.In the first step, alcohol is reacted with urea to produce an alkylcarbamate. In the second step, dialkyl carbonate is produced by reactingfurther the alkyl carbamate with alcohol at temperatures higher than thefirst step. The reactions are carried out by employing an autoclavereactor. However, when methanol is reacted with methyl carbamate orurea, N-alkyl by-products such as N-methyl methyl carbamate (N-MMC) andN-alkyl urea are also produced according to the following reactions:

The dialkyl carbonate is present in the reactor in an amount between 1and 3 weight % based on total carbamate and alcohol content of thereactor solution to minimize the formation of the by-products.

In U.S. Pat. No. 6,010,976, dimethyl carbonate (DMC) is synthesized fromurea and methanol in high yield in a single step in the presence of highboiling ethers and a novel homogeneous tin complex catalyst.(NH₂)₂CO+2CH₃OH=====>(CH₃O)₂CO+2NH₃

The ether solvent also serves as complexing agent to form the homogenouscomplex catalyst from dibutyltin dimethoxide or oxide in situ.

The separation of materials involved in the DMC processes is veryimportant for the commercial production of DMC for economic reasons. EP0 742 198 AI and U.S. Pat. No. 5,214,185 disclose the separation of DMCfrom a vapor mixture of methanol and DMC by using dimethyl oxalate(DMOX) as extraction solvent. Because of the high melting point of DMOX(54° C.), using DMOX is inconvenient and adds an extra cost to theseparation.

Both urea and alcohols are highly hygroscopic. Urea contains an ammoniumcarbamate impurity. Therefore, water and ammonium carbamate areimpurities in urea and alcohol feed. It has been found that impuritiessuch as water, ammonium carbamate, etc, in the urea and alcohol feedscause catalyst deactivation and line plugging on cold spots in thecooling section (the condenser) for the overhead vapor stream from thereactor. Water causes the deactivation of catalyst containing alkyoxygroups, for example, the methoxy groups on the organotin complexcompound molecules are highly reactive with water molecules resulting inhydrolysis of the bond between the tin atom and oxygen atom of methoxygroup. Ammonium carbamate causes problems for controlling thebackpressure in the dialkyl carbonate producing reactor and plugging thecooling system (condenser) of the product vapor stream from the dialkylreactor, because of the deposition of ammonium carbamate.

SUMMARY OF THE INVENTION

Briefly the present invention is an improved process for production ofdialkyl carbonate comprising the steps of:

-   -   (a) feeding a stream containing urea, alcohol, water and        ammonium carbamate to a first reaction zone:    -   (b) concurrently in said first reaction zone,        -   (i) reacting water with urea to form ammonium carbamate,        -   (ii) decomposing the ammonium carbamate in the feed and the            ammonium carbamate from the reaction of water with urea into            ammonia and carbon dioxide, and    -   (c) removing ammonia, carbon dioxide and said alcohol from said        first reaction zone as a first overheads;    -   (d) removing urea and said alcohol from said first reaction        zone;    -   (e) feeding said urea and said alcohol to a second reaction        zone;    -   (f) reacting said alcohol and urea in the presence of a        homogeneous catalyst comprising an organotin complex compound of        dialkylmethoxide in a high boiling solvent to form dialkyl        carbonate and    -   (g) removing dialkyl carbonate and said alcohol from said second        reaction zone.

Dialkyl carbonates are prepared by reacting alcohols, preferably C₁-C₃alcohols, with urea or alkyl carbamate or both in the presence of acomplex of organotin compound with a high boiling electron donorcompound acting as a solvent, preferably dibutyltin dialkoxide complexcompound and high boiling oxygen containing organic solvent, wherein thereaction is preferably carried out in the reboiler of a distillationstill or a stirred tank reactor with concurrent distillation of thedialkyl carbonate. The urea and alcohol feeds are purified by removingwater and ammonium carbamates, N-alkylated by-product and a minorfraction of alkylcarbamate.

The water is removed by reacting it with urea in a prereactor having apreliminary reaction zone. Ammonium carbamate is removed bydecomposition to ammonia and carbon dioxide in the prereactor. Inaddition urea is partially and selectively converted to alkyl carbamatein the prereactor which results in a faster reaction rate in the primaryreactor, a reduction of alcohol recycle to the primary reactor from thedialkyl carbonate recovery unit or column and a higher concentration ofdialkyl carbonate in the overhead stream from the primary reactor. Ahigher concentration of dialkyl carbonate in the overhead stream fromthe primary reactor reduces the cost of separation of the dialkylcarbonate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified flow diagram of one embodiment for producing DMCaccording to the present invention.

FIG. 2 is a simplified flow diagram of a DMC separation unit.

FIG. 3 is a schematic representation of a one embodiment of producingDEC according to the present invention.

FIG. 4 is a simplified flow diagram of a reaction/distillation columnreactor embodiment for the present process.

FIG. is 5 a simplified flow diagram of a catalytic stirred tank reactorwith an attached distillation column embodiment for the present process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The water impurity in the urea and alcohol feeds is removed by reactingthe water with urea in a prereactor while ammonium carbamate is removedby decomposing it to ammonia and carbon dioxide in the prereactor. Theprereactor must be operated under favorable conditions for thedecomposition such that the ammonia and carbon dioxide may be removed asvapors. If the decomposition is incomplete the unconverted ammoniumcarbamate will enter the primary reactor and be converted to urea andwater as it decomposes to ammonia and carbon dioxide causing thedeactivation of the catalyst. Urea is partially converted to alkylcarbamate in the prereactor. The following necessary reactions occur inthe prereactor:

Since the above four are equilibrium reactions and must occursimultaneously in the preliminary reaction zone in the prereactor,controlling the temperature and pressures of the prereactor and theprimary reactor is important. The reactions (1), (2), (3) and (4) arecarried out in the prereactor at a temperature from 200 to 380° F.,preferably from 250 to 350° F. in liquid phase in the prereactor. Thepreferred range of the overhead pressure of the prereactor is from about30 to 300 psig. However, the overhead pressure is determined mainly bythe desired temperature of the prereactor column and the composition ofthe liquid in the reactor. Reaction (4) proceeds to equilibrium in theabsence of a catalyst, but the reaction is faster in the presence of acatalyst such as dibutyltin dialkoxide complex catalyst and weaklyacidic or basic heterogeneous catalyst such as zinc oxide, tin oxide,titanium oxide, zirconium oxide, molybdenum oxide, talcite, calciumcarbonate, zinc carbonate hydroxide, zirconium carbonate hydroxide, etc.supported on a inert support such as silica, high temperature (>850° C.)calcined alumina. The preferred concentration of urea in the liquidphase in the preliminary reaction zone under given conditions is lessthan about 80 wt. %, preferably 50 wt. %. The partial pressures ofammonia and carbon dioxide must be kept below the decomposition pressureof ammonium carbamate to allow the decomposition of ammonium carbamate.It is also highly desirable to effectively remove the products, ammoniaand carbon dioxide, from the preliminary reaction zone of theprereactor, preferably as a vapor mixture along with alcohol and anyinert stripping gas employed as an option. Alcohol vapor may be used asthe sole stripping gas if desired. Thus, the improvement is in a processfor the production of dialkyl carbonates by the reaction of reactantscomprising urea and alcohol having water and ammonium carbamate asimpurities comprising the steps of:

-   -   (a) feeding reactants comprising urea and alcohol to a primary        reaction zone;    -   (b) feeding an organotin compound and a high boiling electron        donor atom containing solvent to said primary reaction zone; and    -   (c) concurrently in said primary reaction zone        -   (i) reacting alcohol and urea in the presence of said            organotin compound and said high boiling electron donor atom            containing solvent to produce dialkyl carbonate; and        -   (ii) removing the dialkyl carbonate and ammonia from said            primary reaction zone as vapor,    -   wherein the improvement is the use of a preliminary reaction        zone before the primary reaction zone to remove water, and        ammonium carbamate from said reactants, by feeding the        reactants, first to the preliminary reaction zone under        conditions to react said water with urea to form ammonium        carbamate and decompose ammonium carbamate to ammonia and carbon        dioxide and removing the ammonia and carbon dioxide from said        reactants prior to feeding the reactants in step (b), preferably        at a temperature in the range from 200 to 380° F., more        preferably from 250 to 350° F. and preferably in liquid phase.        Preferably a portion of the urea and alcohol react to form alky        carbamate in the preliminary reaction zone.

A preferred embodiment of the process for the production of dialkylcarbonates comprises the steps of:

-   -   (a) feeding urea, C₁-C₃ alcohol prereaction zone;        -   (i) cleaning the impurities in feeds in the prereactor;        -   (ii) removing ammonia, carbon dioxide and alcohol as vapor            stream;        -   (iii) reacting a portion of urea and alcohol to alkyl            carbamate; and        -   (iv) removing a liquid stream containing alkyl carbamate,            urea and alcohol to introduce to a primary reaction zone;    -   (b) feeding an organotin compound and a high boiling electron        donor atom containing solvent to said primary reaction zone;    -   (c) concurrently in said primary reaction zone        -   (i) reacting C₁-C₃ alcohol, urea and alkyl carbamate in the            presence of said organotin compound and said high boiling            electron donor atom containing solvent to produce dialkyl            carbonate; and        -   (ii) removing the dialkyl carbonate and ammonia, ether,            carbon dioxide, N-alkyl alkyl carbamate and alkyl carbamate            from said primary reaction zone as vapor; and    -   (d) converting N-alkyl alkyl carbamate separated from the vapor        stream from primary reactor and in a small slipstream of liquid        reaction medium from primary reaction zone to said heterocyclic        compounds ((RNCO)₃, where R is H or C_(n)H_(2n+1) and n=1, 2        or 3) in said third clean-up reaction zone and converting alkyl        carbamate to dialkyl carbonate;        -   (i) removing heterocyclic compounds in the stream from the            third reaction zone as solids;        -   (ii) returning the reaming liquid stream to the primary            reaction zone and the clean-up reaction zone, and        -   (iii) removing ammonia, alcohol and dialkyl carbonate as            overhead vapor stream

This embodiment provides improvements which include the use of apreliminary reaction zone to remove water, and ammonium carbamate fromsaid urea and alcohol, preferably at a temperature in the range from 200to 380° F., more preferably from 250 to 350° F. and preferably in liquidphase and the use of a clean-up reaction zone to convert the by-productN-alkyl alkyl carbamate to heterocyclic compounds at a temperature inthe range from 300 to 400° F. in liquid phase to remove as solid fromthe system.

The preferred prereactor is double diameter tower reactor (widerdiameter at lower section). The urea feed solution in alcohol isintroduced to the column prereactor at the middle section of narrowerdiameter upper section. The ammonium carbamate in the urea feed isdecomposed to ammonia and carbon dioxide. The temperature of the columnis maintained at a temperature from about 200 to about 380° F. underfrom 50 to 350 psig. The light reaction products, ammonia and carbondioxide, are removed from the column as an overhead vapor stream alongwith alcohol vapor. Urea in the feed stream is, at least, partiallyconverted to alkyl carbamate in this prereactor. This reaction isexothermic. The conversion of urea is higher than 10%, preferably higherthan 50%. The conversion of urea to alkyl carbamate can be carried outin the absence of the complex catalyst. But with the catalyst theconversion rate is faster.

Removing water and ammonium carbamate impurities in the feed streamssolves the problems associated with keeping the catalyst in the activestate, controlling overhead pressure of the distillation column, andplugging of cooling area of overhead vapor stream from primary reactorby deposition of ammonium carbamate. Cleaning-up of the impurities infeeds is carried out in a prereactor, which is a double diameterdistillation column reactor. Removing the impurities in the feed streamsis the primary objective of the prereactor. Further improvement is madeby at least partially converting urea to alkyl carbamate in theprereactor, which results in faster reaction rate of producing dialkylcarbonate in the primary reactor, a reduction of alcohol recycle to theprimary reactor from dialkyl carbonate recovery unit because of higherconcentration of dialkyl carbonate in the overhead stream from theprimary reactor. In making dialkyl carbonate, a higher concentration ofdialkyl carbonate in the overhead stream from the primary reactorreduces the separation cost of dialkyl carbonate.

The primary reactor, where dialkyl carbonate is formed, is stirred tankreactor equipped with a heat exchanger to recover the latent heat of theproduct vapor stream from the primary reactor. The recovered heat isused to recycle alcohol from alcohol recovery column to primary reactor.It is not necessary, but optional that the liquid reaction medium ismechanically stirred. In the present invention, thereaction/distillation column of the primary reactor is operatedunconventionally so that the undesired N-alkylated by-products areremoved from the liquid reaction zone as parts of overhead productstream, which allows maintaining the by-products at the minimal level sothat the reactor can be operated at a constant liquid level withoutfilling the liquid reaction zone with undesired by-products for anextended period of reactor operation without any interruption. This ishighly desirable for the successful commercial production of dialkylcarbonate. Lower concentrations of urea, alkyl carbamate and dialkylcarbonate in the liquid medium are utilized to minimize the rate of theformation of N-alkylated by-products, which is accomplished by using ahigher concentration of high boiling solvent such as triglyme. However,if the alkyl carbamate concentration is too low an unacceptably lowspace yield of DMC can occur.

To avoid accumulation of the by-products such as N-alkyl alkyl carbamateand heterocyclic compounds in the primary reactor, it was discoveredthat the N-alkyl alkyl carbamate could continuously be distilled offfrom the liquid reaction medium, while concurrently performing thedialkyl carbonate producing reaction, by controlling both temperatureand pressure of the distillation column of the primary reactor vaporstream, and be converted to heterocyclic compounds, which can be removedas solids from the system. In other words, it was discovered that steadyconcentrations of N-alkyl alkyl carbamate and heterocyclic compounds ina given liquid reaction volume of the primary reaction zone could bemaintained under a steady station reactor operation condition. It wasalso discovered that maintaining the skin temperature of any internalportion of the primary reactor at a temperature below about 550° F.,preferably below 450° F. is highly desirable to minimize the formationof heterocyclic compounds in the primary reaction zone. The conversionof N-alkyl alkyl carbamate to heterocyclic compounds is carried out byemploying a third clean-up reaction zone.

The preferred clean-up reactor for this purpose is a stirred tankreactor equipped with an attached distillation column, a condenser andreflux drum. The by-product N-alkyl alkyl carbamate produced byalkylation of alkyl carbamate with dialkyl carbonate in the primaryreaction zone is continuously removed as a part of overhead vapor streamalong with other products by operating the vapor temperature from theprimary reaction zone at a column temperature higher than about 255° F.,preferably higher than 265° F.

The N-alkyl alkyl carbamate is separated from the overhead stream fromthe primary reactor and introduced to the clean-up reactor. The clean-upreactor is preferably operated at a temperature of the liquid reactionmedium in range from 330 to 400° F. It is important that the columntemperature and the column overhead pressure are controlled so that theoverhead vapor stream does not contain N-alkyl alkyl carbamate. Ingeneral, the clean-up reactor is operated at a reaction temperature andan overhead pressure at, at least, 2° F. and 5 psig higher than those ofthe primary reactor.

The technique of removing the by-products disclosed in this inventioncan be extended to the prior art such as U.S. Pat. No. 6,359,163 B2(2002) and WO 95/17369 (1995), U.S. Pat. No. 6,031,122 (2000), and EP1167339 (2002) producing dialkyl carbonate from urea and an alcoholregardless whether a solvent is used or not in the primary reactor andthe clean-up reactor.

For the production of heavier alkyl carbonate such as dipropylcarbonate, dibutyl carbonate, etc., removing N-alkyl alkyl carbamatefrom the primary reactor as parts of the overhead stream becomesdifficult. Therefore, a liquid slip stream is taken out of the primaryreactor in larger quantity to the clean-up reactor. N-alkyl alkylcarbamate in this slip stream is converted to separable high boilingpoint materials in the clean-up reactor as disclosed in this invention.After removing high boiling waste materials in the bottom stream fromthe clean-up reactor, the remaining liquid stream may be returned to theprimary reactor and the clean-up reactor.

Various physical devices may be utilized as the prereactor. Theseinclude a distillation column reactor, a stirred tank reactor, bubblereactor, tubular reactor, boiling point reactor or any combinationthereof. The preferred device is a distillation column reactor, in whichthe reactions are carried out under reaction/distillation conditions.Despite the equilibrium nature of the reactions (1), (2) and (3),employing the distillation column reactor allows driving the threereactions to the right, that is, complete removal of water and ammoniumcarbamate in the feed streams. Urea is partially converted to alkylcarbamate in the prereactor according to equilibrium reaction (4). Byremoving ammonia from the reaction zone as an overhead gas mixture, thereaction (4) can be forced to the right side of the reaction as well.The partial conversion of urea to alkyl carbamate increases the rate ofconversion of alkyl carbamate to dialkyl carbonate in the primaryreactor and results in a higher concentration of dialkyl carbonate inthe overhead stream from the primary reactor because the reaction ofurea with alcohol to produce dialkyl carbonate occurs in two steps andreaction (4) is the first step.

The dialkyl carbonate forming reaction is as follows:

Reaction (5) is carried out in the primary reactor in the presence of ahigh boiling solvent in the reaction/distillation mode to create afavorable condition for fast removal of dialkyl carbonate from thereaction medium as soon as it is produced. The rate of forming dialkylcarbonate in the primary reactor is more sensitive to the concentrationof ammonia in reaction medium in the primary reactor than the rate offorming alkyl carbamate in the prereactor due to the chemicalthermodynamics. The rate of forming dialkyl carbonate becomes faster,if, at a given concentration of alkyl carbamate, there is a lowerammonia concentration in the liquid reaction medium in the primaryreactor. The temperature of the reaction medium in the primary reactoris from about 300 to about 450° F., preferably from about 320 to 400°F., most preferably from about 330 to 360° F. under a pressure fromabout ambient to 150 psig, preferably from 30 to 120 psig. Anycombination of desired temperature and pressure, which results in highselectivity of dialkyl carbonate, can be obtainable by choosing a properhigh boiling solvent and controlling the concentration of the solvent inthe primary reactor. It is highly desirable that the primary reactor beoperated to have the temperature of the overhead vapor at least about300° F., preferably higher than about 320° F. for the recovery of thelatent heat of the overhead vapor stream to be used for the alcoholrecycle as super heated alcohol vapor to the primary reactor andprereactor.

Using high boiling solvent in the primary reactor allows carrying outthe reaction under low pressure and low concentration of carbamate inthe liquid reaction medium. Lower pressure favors faster removal ofdialkyl carbonate from the liquid reaction medium to the vapor phase,resulting in lower concentrations of dialkyl carbonate in the liquidreaction medium. The lower the concentrations of dialkyl carbonate andcarbamate/urea in the liquid reaction medium, the lower the undesiredby-products associated to N-alkylation and the decomposition products ofurea, alkyl carbamate and N-alkylated products in the primary reactor.The preferred solvent for the synthesis of dialkyl carbonates shouldhave the following properties: (1) The solvent should boil at least, 20°F. higher temperature than the boiling point of the dialkyl carbonateproduct; and (2) It should not form an azeotropic mixture with dialkylcarbonate. The examples of such a solvent are high boiling ethers,ketones, hydrocarbons, and esters or mixtures of these; triethyleneglycol dimethyl ether, tetraethylene glycol dialkyl ether, anisol,dimethoxy benzene, dimethoxy toluene, alkyl oxalate, decaline,tetraline, xylene, decane, etc. or mixtures of these.

A super heated alcohol vapor stream is directly introduced into theliquid reaction zone to supply the heat of reaction for the conversionof alkyl carbamate to dialkyl carbonate which is a slightly endothermicreaction, and strip off dialkyl carbonate and ammonia from the liquidreaction medium as soon as dialkyl carbonate is produced. The desiredtotal concentration of alkyl carbamate and urea combined in the reactionmedium is from about 10 to about 60 wt. %, preferably from about 15 toabout 50 wt. % of the total materials in the liquid reaction medium. Thedesired concentration of dialkyl carbonate in the reaction medium isfrom about 0.5 to about 12 wt. %, preferably from about 2 to about 9 wt.% based on the total content of the liquid reaction medium. The moleratio of alkyl carbamate to alcohol in the liquid reaction medium isfrom 0.2:1 to 2:1, preferably from 0.3:1 to 1.5:1. The concentration oforganotin complex catalyst is from about 2 to about 20 wt. % tin,preferably from 5 to about 17 wt. % tin based on the total content ofall the materials in the liquid reaction zone in the primary reactor.Note that the catalyst also catalyzes the undesired side-reactionsdiscussed above. Carrying out the reaction at lower temperature reducesthe side-reactions. However, the rate of producing dialkyl carbonate isalso lower, which may not be acceptable for the commercial production ofDMC. The desired concentration of high boiling solvent in the reactionmedium in the primary reactor is from about 2 to about 65 wt. %,preferably from 2.5 to 55 wt. % of the total material in the reactionmedium.

The working catalyst under steady state reaction conditions is anorganotin complex catalyst system derived from the organotin complexcompound of dialkyltin dialkoxide, R′_(4-n)Sn(OR)_(n).xL (Where R′ isalkyl group, aryl or aralkyl group; R=alkyl; n=1 or 2; x=1 or 2; L iselectron donor atom containing monodentate or bidentate liqand). Theexamples of L are electron donor ligand molecules such as ethers,esters, ketones, aldehydes, organic phosphines or mixtures of these;triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether,dimethyl oxalate, dimethyl malonate, dimethyl succinate, anisol,dimethoxy benzene, dimethoxy toluene, ethylene glycol, catecol,1,4-dioxane-2,3-diol, 2-methyltetrahydrfuran-3-one, 2,3-pentanedione,2,4-pentanedione, 3-methyltetrahydropyran, triphenylphosphine, etc. Thehomogeneous catalyst system is a quasi-equilibrium mixture of variousorganotin species. Suitable catalysts of this type and their method ofmanufacture are described in U.S. Pat. Nos. 6,010,976 and 6,392,078which are incorporated herein in their entirety.

The working catalyst system under the steady state reaction condition isa mixture of various soluble organotin monomer, dimer and oligomerspecies, which are produced by a number of possible reactions. Thesevarious organotin catalyst species are more or less in quasi-equilibriumstate under a given reaction condition. Dialkyltin oxide, dialkyltinhalides, dialkyltin bis(acetylacetonate) and dialkyltin carboxylatessuch as dibutyltin diacetate, dialkyltin oxalate, dibutyltin malonate,dibutyltin diacetate, dibutyltin bis(acetylacetonate) etc. can be usedto form the soluble tin complex catalyst species in situ at the start-upof the primary reactor by reacting with alcohol in the presence of ahigh boiling solvent such as triglyme. The alkyl groups attached to tinatoms can be the same or different. For example, the catalyst precursorcan be dibutyltin, butylbenzyltin, butylphenyltin, butyloctyltin ordi-2-phenylethyltin dialkoxide, dihalides, hydroxyhalide, diacetate oroxide. Water, carboxylic acid or hydrochloric acid co-products arecontinuously removed from the liquid reaction medium as an overheadvapor stream in the presence of high boiling solvent under low pressure.The suitable temperature for the catalyst forming reaction is from about200 to about 400° F. and pressure from about ambient to 150 psig. In apreferred embodiment, methanol, ethanol or propanol depending on theintended dialkyl carbonate product is continuously pumped into theprimary reactor. Either methanol or ethanol is acceptable for theproduction of MEC. The catalyst forming reaction is advantageouslycarried out in the presence of dilute alkyl carbamate, N-alkyl alkylcarbamate or dialkyl solution in the primary reactor. It is understoodthat during this catalyst forming reaction the operation of thedistillation column should be operated under conditions, which allowremoving co-products water, carboxylic acid or hydrogen chloride as anoverhead stream along with alcohol as an overhead product from thereaction zone. For the dimethyl carbonate production, the solubleorganotin complex catalyst system is formed by simply mixing dibutyltindimethoxide with triglyme (as a complex agent for the formation oforganotin complex catalyst) and methanol in the primary reactor prior toinitiating the dialkyl carbonate forming reaction in the presence of ahigh boiling solvent. For diethyl carbonate production, the solubleorganotin complex catalyst system is preferably formed by usingdibutyltin dimethoxide with triglyme and ethanol in a primary reactor.As the reaction proceeds, the methoxy groups on the catalyst arereplaced with ethoxy groups.

Referring now to FIG. 1 there is shown a simplified flow diagram of oneembodiment of the invention. FIG. 1 illustrates the flow diagram of theimproved process excluding the DMC separation unit. The DMC separationunit is illustrated in FIG. 2. A reaction/distillation column reactor111 is used as the prereactor to remove the impurities in the feedstreams and for the partial conversion of urea to methyl carbamate. Ureasolution is prepared in the drum 131 by mixing urea feed 1 and methanolstream 3. The methanol stream is comprised of the fresh methanol feedstream 2 and a methanol recycle stream 4, which is a portion of themethanol recycle stream 30 from the DMC separation unit. The methanolrecycle stream 30 from the DMC separation unit (shown in FIG. 2) splitsinto three streams (4 and 33 via 31, and 32) to be used for thepreparation of urea solution, the primary reactor 112 and the clean-upreactor 113. The urea solution feed 5 from the drum 131 is introduced tothe middle of upper narrower column section of the double diameter towerreactor 111. The reactor 111 serves as a prereactor to clean up theimpurities (water and ammonium carbamate) in the methanol and ureafeeds, and to partially covert urea to MC. A vapor stream 6 from theprereactor 111 is composed of ammonia, carbon dioxide and methanol. Thecleaned mixed solution of MC and urea in methanol is removed from theprereactor 111 as bottom stream 7 and combined with the recycle liquidstream 22 from the cooling/filter system 133 via the line 20 to thestream 8. Liquid recycle stream 20 from the cooling/filter system 133splits into two streams (21 and 22) to recycle to primary reactor 112and clean-up reactor 113. The combined stream 8 is introduced to theprimary reactor (stirred tank reactor or optionally bubble columnreactor) 112. The recycle methanol stream 33 from the DMC separationunit is introduced into the primary reactor 112 as super heated methanolvapor. The overhead recycle stream 14 from the flash column 132 isintroduced to the primary reactor. The stream 14 is comprised of mostlyMC and minor amounts of N-MMC and methanol. The overhead vapor stream 9from the primary reactor 112 is comprised of ammonia, CO₂, dimethylether, methanol, DMC, MC, N-MMC, TG and a small amount of organotincatalyst. This overhead stream 9 from the primary reactor 112 iscombined with the overhead stream 17 from the clean-up stirred tankreactor 113 to the stream 10. The stream 17 is comprised of ammonia,CO₂, dimethyl ether, methanol and DMC. The combined stream 10 isintroduced to the distillation column 151 to separate the components,ammonia, CO₂, dimethyl ether, methanol and DMC from the rest of heaviercomponents in the stream. Overhead stream 12 from the column 151 iscombined with the overhead stream 6 from the prereactor reactor 111 tothe stream 23. The combined stream 23 is introduced to the distillationcolumn 134. Overhead stream 25 from the column 134 is sent to theammonium carbamate removing system 135, where the stream 25 is suitablycooled to cause the reaction of CO₂ and ammonia to ammonium carbamateand precipitation of solid ammonium carbamate. Solid ammonium carbamatemay be removed by using a filter or hydroclone. Stream 27 from theammonium carbamate removing system 135 is introduced to the distillationcolumn 136 to recover ammonia. Overhead stream 28 from the column 136 issent to an ammonia storage tank. Bottom stream 29 from the column 136 isorganic waste, mostly composed of dimethyl ether. Bottom stream 24 fromthe column 134 is sent to the DMC separation unit shown in FIG. 2.Stream 24 is comprised of DMC and methanol. Bottom stream 13 from thecolumn 151 is sent to the flash column 132. Stream 13 is comprised ofmethanol, MC, N-MMC, TG and a small amount of catalyst. Bottom stream 15from the flash column 132 is comprised of N-MMC, MC (minor amount), TGand a small amount of organotin catalyst. Stream 15 is combined with asmall liquid slipstream 11 from the primary reactor 112 to the stream16. The combined stream 16 is introduced to the clean-up reactor 113(Optionally the small slipstream 11 from the primary reactor 112 may beintroduced to the lower section of either the column 151 or 132). Bottomstream 18 from the clean-up reactor 113 is chilled to precipitate theheterocyclic compounds such as isocyanuric acid, 1,3,5-trimethyltriazine-2,4,6-trione, etc. in the stream and the precipitate is removedfrom the filtration system 133 as solid through line 19. Liquid filtratestream 20 from the filtration system 133 splits to two streams 21 and 22to recycle back to primary reactor 112 and clean-up reactor 113.

The primary reactor can be a single stirred tank reactor or a multiplestirred tank reactor system depending on the production capacity ofdialkyl carbonate. For example, if the primary reactor is comprised of aseries of three stirred tank reactors, the cleaned alkyl carbamate/ureafeed stream 7 from the prereactor 111 is suitably divided into threestreams and each stream is introduced directly to three reactors. Aliquid reaction stream is withdrawn from each reactor and introduced tothe next reactor. The liquid reaction stream from the third reactor issent to the first reactor. A small slipstream from the liquid reactionstream from the third reactor is combined with the bottom stream 15 fromcolumn 132 to stream 16, which is sent to clean-up reactor 113. Thealcohol recycle stream such as methanol recycle stream 33 from the DMCseparation unit is introduced to the first reactor as super heatedalcohol vapor. The overhead vapor stream from the first stirred tankreactor is introduced to the second reactor. The overhead vapor streamfrom the second reactor is introduced to the third reactor. The contentof dialkyl carbonate in the overhead stream from each reactor increasesas the vapor stream moves from the first reactor to third reactor,resulting in the highest concentration of DMC in the overhead streamfrom the third reactor. Overhead stream 9 from the third reactor iscombined with the overhead stream 17 from clean-up reactor 113 to stream10, which is introduced to the distillation column 151.

Bottom stream 24 from distillation column 134 is sent to the DMCseparation unit (see FIG. 2). This stream contains about 28 wt. % DMC.Stream 24 from column 134 is introduced into the extractive distillationcolumn 137 through line 34. Overhead stream 35, which is comprised ofabout 98 wt. % methanol and about 2 wt. % DMC, is recycled through line30 in FIG. 1. Bottom stream 36 is introduced into extractive solventrecovery column 138. The extractive solvent, anisole, is recovered asbottom stream 38 from column 138 and recycled back to column 137 throughline 38. Overhead stream 37 from column 138 is the product DMC, which issent to a DMC storage tank.

Diethyl carbonate (DEC) can be produced in a similar manner describedabove for the production of DMC. Since the mixture of ethanol and DECdoes not form an azeotrope, the separation of DEC from ethanol can becarried out with a single distillation column.

FIG. 3 illustrates the simplified flow diagram of the process for theproduction of DEC. The urea solution is prepared in drum 141 by mixingurea feed 101 and ethanol stream 103. Ethanol stream 103 is comprised offresh ethanol feed 102 and ethanol recycle stream 155, which is aportion of ethanol recycle stream 130. Ethanol recycle stream 130 iscomprised of overhead stream 117 from distillation column 146 and bottomstream 129 from distillation column 145 (ethanol recovery column).Ethanol recycle stream 130 splits into two streams 152 and 156. Stream152 is recycled to clean-up reactor 123. Stream 156 splits again to twostreams 155 and 157 to recycle to drum 141 and primary reactor 122,respectively. Ethanol recycle stream 157 is introduced to primaryreactor 122. The urea solution 104 from drum 141 is introduced to themiddle of upper narrower column section of the double diameter towerreactor 121. Reactor 121 serves as a prereactor to clean up theimpurities (water and ammonium carbamate) in the feeds, ethanol andurea, and to partially convert urea to EC. Vapor stream 105 fromprereactor 121 is composed of ammonia, carbon dioxide and ethanol. Thecleaned mixed solution is removed from prereactor 121 as bottom stream106. Stream 106 is introduced to primary reactor (stirred tank reactoror optionally bubble column reactor) 122. Recycle ethanol stream 157(this stream is the major portion of the ethanol recycle stream 130) isintroduced into the primary reactor 122 as superheated ethanol vapor. Asmall slipstream 108 from primary reactor 122 is combined with bottomstream 120 from DEC recovery column 147 to form stream 161 and combinedstream 161 is introduced to clean-up reactor 123. Slipstream 108 iscomposed of ethanol, ammonia, diethyl ether, DEC, ethyl carbamate,N-ethyl ethyl carbamate, TG, heterocyclic compounds such as isocyanuricacid, 1,3,5-triethyl triazine-2,4,6-trione, etc and organotin complexcatalyst. Stream 120 is comprised of ethanol, ethyl carbamate, N-ethylcarbamate, TG, and trace amounts of catalyst. Overhead stream 160 fromclean-up reactor 123 is comprised of ammonia, CO₂, diethyl ether,ethanol, and DEC. Bottom stream 159 from the clean-up reactor 123 iscomprised of ethyl carbamate, triglyme, N-ethyl ethyl carbamate,ethanol, heterocyclic compounds and homogeneous organotin catalyst.Bottom stream 159 from reactor 123 is cooled to precipitate heterocycliccompounds in the cooling/filter system 148. The precipitate solidby-product is removed from system 148 through line 124. Liquid stream125 from system 148 splits into two streams 126 and 127 to recycle toprimary reactor 122 and clean-up reactor 123. Overhead stream 107 fromprimary reactor 122 is combined with overhead stream 160 from clean-upreactor 123 to stream 109. Overhead vapor stream 107 from primaryreactor 122 is composed of ammonia, CO₂, diethyl ether, ethanol, ethylcarbamate, N-ethyl ethyl carbamate, DEC, TG and trace amount ofcatalyst. Stream 107 is combined with overhead stream 160 from clean-upreactor 123 to stream 109. Overhead stream 160 is comprised of ammonia,CO₂, diethyl ether, ethanol and DEC. Combined stream 109 is introducedto distillation column 142. Overhead stream 110 from column 142, whichis composed of ammonia, CO₂, diethyl ether and ethanol, is introduced todistillation column 143. Overhead stream 154 from column 143 is cooledto cause the reaction of CO₂ with ammonia to produce ammonium carbamate.Ammonium carbamate is precipitate in liquid ammonia and removed assolids through line 115 from cooling/filter system 144. Liquid ammoniastream 116 from cooling/filter system 144 is sent to an ammonia storagetank. Bottom stream 150 from column 142 is composed of DEC, ethanol,ethyl carbamate, N-ethyl ethyl carbamate, TG and a trace amount ofcatalyst. Stream 150 is introduced to distillation column 146 (the firstethanol recovery column). Overhead ethanol stream 117 from column 146 iscombined with ethanol bottom stream 129 from distillation column 145(the second ethanol recovery column) to ethanol recycle stream 130.Bottom stream 118 from column 146 is introduced to distillation column147 (DEC recovery column). Overhead stream 119 is the product DEC, whichis sent to a DEC storage tank. Bottom stream 120 from column 147 iscombined with a small slipstream 108 from primary reactor 122 andcombined stream 161 is introduced to stirred tank clean-up reactor 123.Overhead stream 160, which is composed of ammonia, CO₂, diethyl ether,ethanol and DEC, is sent to column 142 through line 109. Bottom stream114 from column 143 is sent to distillation column 145 to separatediethyl ether from ethanol in the stream. The overhead ether by-productstream 128 is sent to an ether storage tank. Bottom stream 129 fromcolumn 145 is ethanol, which is recycled to clean-up reactor 123 andprimary reactor 122 through line 130.

A mixed dialkyl carbonate such as methyl ethyl carbonate (MEC) isproduced by using suitable mixtures of methanol and ethanol as feedstreams in place of the methanol or ethanol feed stream to the drum forthe preparation of urea solution. However, the overhead streams from theprimary and clean-up reactors contain some DMC and DEC in addition toMEC. DEC and DMC are separated from the mixture and the exchangereaction of DEC and DMC to MEC is carried out in a separate reactor (notshown). This exchange reaction is carried out by using either aheterogeneous base catalyst such as an alkaline form of zeolites, basictalcite, etc or a homogeneous catalyst such as Group IVB compounds suchas titanium tetraethoxide or ethoxycarbonates or dialkyltin compoundssuch as alkoxide, dialkyltin methoxy alkyl carbonate, dialkyltincarbonate, or the organotin complex catalyst system discussed above inthe absence or presence of a solvent. The suitable solvent will have aboiling point higher than about 265° F. The examples of such a solventare hydrocarbons such as decaline, decane, xylene, diglyme, triglyme,etc or mixtures of these.

FIG. 4 illustrates a reaction/distillation column reactor using a basicheterogeneous catalyst. The DEC feed is introduced to thereaction/distillation column reactor 153 through lines 221 at a positionof the top section of catalyst bed. The DMC feed is introduced to 153 ata position below the catalyst bed through line 222. The overhead stream223 is comprised of mostly DMC and MEC. The stream 223 is sent to MECseparation unit. The temperature of the liquid catalytic reaction zoneis maintained in a range from about 200 to about 450° F., preferablyfrom 235 to 380° F. To prevent the build-up of heavies in the reboilerof column reactor 153, a small amount of bottom is withdrawn throughline 224.

FIG. 5 illustrates a catalytic stirred tank reactor 158 with an attacheddistillation column 162. A homogeneous catalyst such as dibutyltindimethoxide is used. A predetermined amount of a homogeneous catalyst ischarged into reactor 158 prior to carrying out the reaction. A mixedDEC/DMC feed is introduced to reactor 158 through line 301. The overheadstream 303 from 162 is comprised of mostly DMC and a small amount of MECis recycled back to reactor 158. A side draw stream 302, which isconcentrated with MEC, from distillation column 162 is sent todistillation column 163 to separate MEC from DMC. The overhead stream304 from column 163 is recycled back to reactor 158. The bottom MECstream 305 from column 163 is sent to a MEC storage tank. Thetemperature of the liquid catalytic reaction zone is maintained in arange from about 200 to about 450° F., preferably from 235 to 380° F.The range of the overhead pressure of column 162 is from about 20 psigto about 150 psig, preferably from about 25 to about 120 psig. But theoverhead pressure of column 162 is determined by the intendedtemperature of liquid reaction medium in 158, the composition of thereaction medium, and whether a solvent is used or not.

EXAMPLE 1

The reaction of water with urea is carried out in this example. Thefollowing materials are charged in a 500 ml three neck flask equippedwith a magnetic stirrer and water cooled reflux condenser: 229.67 gramstriethylene glycol dimethyl ether (triglyme), 1.58 grams of water, 2.06grams of methanol and 15.89 grams of urea. When the reaction temperatureof the mixture in the flask reaches about 100° C., 3.2 grams ofadditional methanol are added. The reaction of water with urea iscarried out at a temperature from 128 to 140° C. for 0.92 hours undernitrogen blanket. The analysis of the sample taken from the flask arecarried out by GC and HPLC. The analytical results indicate 22.4%conversion of urea and 45.2% conversion of water.

EXAMPLE 2

General Description of the Experiment

A one liter stirred autoclave serves as the reaction zone and reboilerfor the reaction/distillation column reactor, which is connected to a 1inch diameter×3.5 feet long distillation column. The distillation columnhas three zone heaters, which are independently controlled. The overheadvapor stream from the distillation column is diluted with a nitrogenstream (800 cc/min) and then partially cooled to about 200° F. with hotwater in a condenser. The vapor stream from the condenser cooled toambient temperature to prevent the plugging problem of a cold spot andoverhead backpressure regulator. The liquid stream from the condenserflows to a small overhead liquid reflux drum. The temperature of theliquid reflux drum is maintained at ambient temperature. The flow of theliquid product from the overhead reflux drum is monitored with a LFM(liquid flow meter). The liquid stream from the overhead reflux drum andthe cooled vapor stream are combined as product stream from thereaction/distillation reactor. Samples are taken for analyses todetermine the composition of the overhead vapor stream coming out of thecolumn. Also occasionally samples are taken from the reboiler to monitorthe composition of the liquid reaction medium. Whenever the samples aretaken from the reboiler, the make-up solutions are pumped in tocompensate for the loss of triglyme and catalyst. During the operationof the reactor, the liquid level inside the reboiler is maintained at aconstant level. A vertical sight glass is attached to the reboiler forthe visual observation of the liquid level inside the reboiler duringthe operation. Also the reboiler is equipped with a liquid level digitalmonitor for the automatic control of the reactor during the night andweekends for unattended operation.

To carry out the operation of the primary reactor to produce DMC, a MCfeed solution (methyl carbamate in methanol) and a methanol feed arepumped in and combined into a single stream. The combined feed stream ispassed through a prereactor (a vertically mounted tubular reactorup-flow) at 300° F. and 230 psig to remove water in the feed streams andthen introduced to the primary reactor. The temperature of the liquidreaction medium is controlled by adjusting the overhead pressure of thedistillation column and the concentration of high boiling solvent in thereboiler of the distillation column. The products DMC, ammonia and otherlight by-products such as dimethyl ether and CO₂ are boiled off from theliquid medium and carried away along with methanol vapor. The operationof the distillation column is carried out in the unconventional mode toperform partial condensation of the vapor coming out of the liquidmedium in the reboiler without liquid reflux from the overhead refluxdrum by controlling the vapor temperature, which is done by controllingthe zone temperatures of the column with three column zone heaters,while the vapor is coming up the distillation column. It was discoveredthat the unconventional column operation keeps the triglyme solvent inthe reactor and continuously removes the by-product N-MMC along with MCfrom the liquid reaction medium as a part of the overhead stream, whichallows the operation of the reactor for an extended period of time. Itis found that no liquid reflux from the overhead reflux drum is highlypreferred in minimizing the formation of the by-product N-MMC andheterocyclic compounds. It was possible to operate thereaction/distillation column reactor more than 1000 hours withoutinterruption until a high pressure nitrogen valve to the reboiler wasaccidentally opened. Operating the distillation column in theconventional way causes shutdown or removal of materials from thereboiler, because of the overflow of the reboiler due to theaccumulation of the reaction by-products such as N-MMC, cyanuric acidand TTT (1,3,5-trimethyl triazine-2,4,6-trione), etc.

Other critical factors to minimize the side-reactions while maintainingan acceptable DMC production rate are balancing the concentrations ofsolvent and catalyst, the temperature of liquid medium and the overheadcolumn pressure. The range of optimum operation for the reboilertemperature and the overhead column pressure is from about 330 to about355° F. for the reboiler temperature and from about 80 to about 110 psigrespectively.Detailed Description of the Experiment

The reboiler of the distillation column was loaded with the followingmaterials; 285 grams of triglyme, 100 grams of methanol and 100 grams ofdibutyltin dimethoxide. A steady state operation of thereaction/distillation column reactor was obtained, while pumping in the13.3 wt. % MC solution in methanol (˜280 ppm H₂O) at a fixed rate of3.01 ml/min and about 1.92 ml/min of methanol (˜80 ppm H₂O) at 345° F.for the liquid reaction medium in the reboiler, 260° F. for the vaportemperature in the top section of the distillation column, and 90.8 psigfor the overhead column pressure. The flow rate of methanol was adjustedto maintain a constant temperature of 345° F. for the liquid reactionmedium. The stirring rate of the reboiler was 300 rpm. Under thisoperational condition, the overhead product stream was composed ofammonia, dimethyl ether, carbon dioxide, DMC, MC, NMMC, water, unknownsand a trace amount (˜1000 ppb) of catalyst. At 926 hours on stream time,the overheads and the bottoms samples were taken. The analyses of thesesamples are listed in Table 1. The mole ratio of MC/CH₃OH and DMC wt. %based on MC and CH₃OH in the liquid medium in the reactor are 1.01 and4.49 wt. % respectively, which interestingly compare with 2-10 and 1-3wt. % claimed in U.S. Pat. No. 5,561,094 (1996, EXXON Chem). The resultof the experiment corresponds to better than 95 mole % of MC to DMC.This experimental data is used to carry out computer simulation for theprocess design. TABLE 1 Sample Analysis (wt. %) OVHD BTM CO₂ 0.10 0.04NH₃ 0.73 0.00 (CH₃)2O 0.04 0.00 Methanol 90.45 11.02 DMC 8.15 1.67 MC0.36 26.17 N-MMC 0.17 1.88 TG 0.00 40.89 Unknown 0.01 0.31 TTT 0.00 3.66Water (ppm) 87 — Sn (ppm) ˜1 14.35**as dibutyltin dimethoxide

EXAMPLE 3

A one liter stirred reactor (autoclave) with a distillation column wasused to remove impurities in an 8.03 wt. % urea solution in methanol andconvert urea to methyl carbamate. No catalyst was charged to thereactor. The experiment was carried out at 315° F. under 200 psig and328° F. under 230 psig by pumping in the urea solution into the reactorat 4 ml/min with the constant bottom flow rate at 2 ml/min for 27 hoursand 3 ml/min with the constant bottom flow rate at 1.5 ml/min to the end(146 hours on stream time) of the run. The distillation column isoperated with overhead reflux. During the operation, the overhead flowwas adjusted to maintain a constant liquid level (50% full) in theautoclave. The column operation was done with overhead reflux from theoverhead reflux drum. The MC concentration in the bottom stream from theautoclave was about 20% on average, which corresponds to about 97%conversion of urea to MC. The urea feed contained about 2000 ppm water.The bottom products contained 375 ppm water at 315° F. and 300 ppm waterat 328° F. on average.

EXAMPLE 4

The purpose of this experiment is demonstrating a primary reactorsystem, which is composed of multiple reactors. The same experimentalset-up in Example 2 was used to demonstrate the performance of thesecond primary reactor. The experiment was carried out in the similarmanner to the Experiment 2. The present example differs from the Example2 in that the 8 wt. % DMC solution in methanol is used herein in theplace of pure methanol in the Experiment 2 and a slightly lower overheadpressure (88 psig) in the present distillation column. The reboiler ofthe distillation column was loaded with the following materials; 285grams of triglyme, 40 grams of methanol and 100 grams of dibutyltindimethoxide. A steady state operation of the distillation column reactorwas obtained, while pumping in MC solution and DMC-methanol solution tothe reactor. The reactor operation was continued for more than 1500hours without interruption at 345° F. for the liquid reaction medium inthe reboiler, the distillation column temperature of ˜278° F., and 88psig for the overhead column pressure. The average compositions of theoverhead and bottom products from the reactor during the 54 hours from1428 hours to 1482 hours of on-stream-time are listed in Table 2. Duringthis period, the pumping rate of a 22.5 wt. % MC solution (˜590 ppm H₂O)was fixed at 1.97 ml/min and the pumping rate of a 8 wt. % DMC solution(80 ppm H₂O) was about 3.2 ml/min at 345° F. The mole ratio of MC/CH₃OHand DMC wt. % based on MC and CH₃OH in the liquid medium in the reactorare 0.915 and 6.40 wt. % respectively. The result of the experimentcorresponds to better than 93 mole % of MC to DMC. TABLE 2 SampleAnalysis (wt. %) OVHD BTM CO₂ tr — NH₃ 0.89 — (CH₃)₂O 0.06 — Methanol84.91 13.32 DMC 12.35 2.68 MC 1.41 28.58 N-MMC 0.36 2.28 TG 0 36.55 HC*0 3.77 Catalyst** 12.83 Water (ppm) 39 — Sn (ppm) 0.6 —*HC; isocyanic acid and 1,3,5-trimethyl triazine-2,4,6-trione**as dibutyltin dimethoxide

EXAMPLE 5

The purpose of this experiment is demonstrating the production ofdiethyl carbonate (DEC). DEC was produced by reacting ethyl carbamate(EC) with ethanol. The experiment was carried out in the similar mannerto the Example 2. An ethyl carbamate solution in ethanol and ethanolwere used in place of MC solution and methanol respectively in Example2. The reboiler of the distillation column was loaded with the followingmaterials; 180 grams of triglyme, 100 grams of ethanol and 100 grams ofdibutyltin dimethoxide. A steady state operation of the distillationcolumn reactor was obtained, while pumping in an ethyl carbamate (EC)solution at a constant flow rate and adjusting the ethanol pumping rateto maintain a constant temperature of the liquid reaction medium. Thereactor operation was continued for 340 hours without interruption at˜345° F. of the liquid reaction medium in the reboiler, the distillationcolumn temperature of ˜282° F., and a constant overhead pressure of 66psig with an autoclave stirring rate of 300 rpm. The pumping rate of a15.35 wt. % EC solution (˜275 ppm H₂O) was fixed at 2.69 ml/min and theaverage pumping rate of ethanol (˜106 ppm H₂O) was 2.36 ml/min. Theoverhead vapor stream at the top of the column was mixed with nitrogendilution gas (600 cc/min) and then cooled to about 200° F. in a watercooled condenser. The average compositions of the overhead products andthe composition of the bottom products for the entire run are listed inTable 3. 74.2 grams of solid material were removed from the reactor atthe end run, which was a mixture of heterocyclic compounds and contained670 ppm Sn by weight. The analysis of the bottom product in Table 3indicates that the mole ratio of EC/C₂H₅OH and DEC wt. % based on EC andethanol in the liquid medium in the reactor was 0.939 and 11.08 wt. %respectively. The mass balance and urea mole balance for the entire runwere 102% and 101%, respectively. The run result indicates 57.5%conversion of EC and 91 mole % selectivity of EC to DEC. Theexperimental result translates to DEC space yield of 1.60 lb/h/ft³.TABLE 3 Sample Analysis (wt. %) OVHD BTM CO₂ tr — NH₃ 0.50 — Ether 0.020.11 Ethanol 90.21 17.60 DEC 7.30 5.50 EC 1.74 32.00 N-EEC 0.15 1.75 TG0.04 21.05 Unknown 0.04 9.34 HC* 0.00 0.95 Catalyst** — 11.70 Water(ppm) 148 — Sn (ppm) 1.7 —*HC; isoycanic acid and 1,3,5-triethyl triazine-2,4,6-trione**as dibutyltin dimethoxide

1. A process for the production of dialkyl carbonate comprising thesteps of: (a) feeding a stream comprising urea, alcohol, water andammonium carbamate to a first reaction zone: (b) concurrently in saidfirst reaction zone, (i) reacting water with urea to form ammoniumcarbamate, (ii) decomposing ammonium carbamate into ammonia and carbondioxide, and (c) removing ammonia, carbon dioxide and alcohol from saidfirst reaction zone; (d) removing urea and alcohol from said firstreaction zone; (e) feeding said urea and alcohol to a second reactionzone; (f) reacting alcohol and urea in the presence of a homogeneouscatalyst comprising an organotin complex compound of dialkylalkoxide ina high boiling solvent to form dialkyl carbonate and (g) removingdialkyl carbonate and alcohol from said second reaction zone.
 2. Theprocess according to claim 1 wherein alcohol and urea react to form alkycarbamate in said first reaction zone.
 3. The process according to claim1 wherein said alcohol is a C₁-C₃ alcohol.
 4. The process according toclaim 1 comprising: (a) feeding said stream containing urea, methanol,water and ammonium carbamate to a first reaction zone: (b) concurrentlyin said first reaction/distillation zone, (i) reacting water with ureato form ammonium carbamate, (ii) decomposing the ammonium carbamate inthe feed and the ammonium carbamate from the reaction of water with ureainto ammonia and carbon dioxide, and (iii) separating the ammonia,carbon monoxide and methanol from the urea and by distillation; (c)removing ammonia, carbon dioxide and methanol from said firstreaction/distillation zone as a first overheads; (d) removing urea andmethanol from said first reaction/distillation zone as a first bottoms;(e) feeding said first bottoms and methanol to a secondreaction/distillation zone; (f) concurrently in said secondreaction/distillation zone, (i) reacting methanol and urea in thepresence of a homogeneous catalyst comprising an organotin complexcompound of dialkylmethoxide in a high boiling solvent to form dimethylcarbonate and (ii) separating dimethyl carbonate and ammonia from thehomogeneous catalyst by distillation. (g) removing dimethyl carbonateand methanol from said second reaction/distillation zone as a secondoverheads; and (h) removing a second bottoms from said seconddistillation column reactor.
 5. The process according to claim 4 whereinthe dimethyl carbonate in said second overheads is separated from themethanol by extractive distillation.
 6. The process according to claim 4wherein an inert diluent is added to said first overheads.
 7. Theprocess according to claim 4 wherein the methanol in said firstoverheads is condensed and a portion of said condensed methanol isreturned to near the top of said first distillation column reactor asreflux and the remainder of said condensed methanol is returned to thelower section of said first distillation column reactor.
 8. The processaccording to claim 4 wherein a first portion of said second bottoms isfed to said first distillation column reactor, a second portion of saidsecond bottoms is recycled to said second distillation column and athird portion of said second bottoms is fed to a third distillationcolumn reactor for catalyst regeneration and heavies cleanup.
 9. Theprocess according to claim 4 wherein said second overheads is condensedand a portion of said condensed second overheads is returned to saidsecond distillation column reactor as reflux.
 10. A process for theproduction of dimethyl carbonate comprising the steps of: (a) feeding astream containing urea, methanol, water and ammonium carbamate to afirst distillation column reactor: (b) concurrently in said firstdistillation column reactor, (i) reacting a portion of said urea with aportion of said methanol to produce methyl carbamate, (ii) reactingwater with urea to form ammonium carbamate, (iii) decomposing theammonium carbamate in the feed and the ammonium carbamate from thereaction of water with urea into ammonia and carbon dioxide, and (iv)separating the ammonia, carbon monoxide and methanol from the urea andmethyl carbamate by distillation; (c) removing ammonia, carbon dioxideand methanol from said first distillation column reactor as a firstoverheads; (d) removing urea and methyl carbamate from said distillationcolumn reactor as a first bottoms; (e) feeding said first bottoms andmethanol to a second distillation column reactor; (f) concurrently insaid second distillation column reactor, (i) reacting methanol and ureain the presence of a homogeneous catalyst comprising an organotincomplex compound of dialkylmethoxide in a high boiling solvent to formdimethyl carbonate and (ii) separating dimethyl carbonate and ammoniafrom the homogeneous catalyst by distillation; (g) removing dimethylcarbonate and methanol from said second distillation column reactor as asecond overheads; (h) removing homogeneous catalyst from said seconddistillation column reactor as a second bottoms; (i) separating thedimethyl carbonate from the methanol in said second overheads byextractive distillation; and (j) feeding a first portion of said secondbottoms to said first distillation column reactor; (k) feeding a secondportion of said second bottoms to a third distillation column reactorwhere the catalyst is regenerated and cleaned up of heavies; and (l)recycling a third portion of said second bottoms to said seconddistillation column reactor.
 11. in a process for the production ofdialkyl carbonates by the reaction of reactants comprising urea andalcohol having water and ammonium carbonate as impurities comprising thesteps of: (a) feeding reactants comprising urea and alcohol to a primaryreaction zone; (b) feeding an organotin compound and a high boilingelectron donor atom containing solvent to said primary reaction zone;and (c) concurrently in said primary reaction zone (i) reacting alcoholand urea in the presence of said organotin compound and said highboiling electron donor atom containing solvent to produce dialkylcarbonate; and (ii) removing the dialkyl carbonate and ammonia from saidprimary reaction zone as vapor, wherein the improvement is the use of apreliminary reaction zone before the primary reaction zone to removewater, and ammonium carbamate from said reactants, by feeding thereactants, first to the preliminary reaction zone under conditions toreact said water with urea to form ammonium carbamate and decomposeammonium carbamate to ammonia and carbon dioxide and removing theammonia and carbon dioxide from said reactants prior to feeding thereactants in step (a).
 12. The process according to claim 11 where thetemperature of the preliminary reaction zone is at a temperature in therange from 200 to 380° F. in liquid phase.
 13. The process according toclaim 12 where the temperature of the preliminary reaction zone is inthe range of from 250 to 350° F.
 14. The process according to claim 11wherein a portion of the methanol feed reacts with a portion of the ureafeed to form methyl carbamate in the preliminary reaction zone.
 15. Theprocess according to claim 11 wherein the preliminary reaction zone andprimary reaction zone are operated under distillation conditions. 16.The process according to claim 11 wherein the water is removed from thefeeds by reacting it with urea to form ammonium carbamate which in turndecomposes to ammonia and carbon dioxide.
 17. The process according toclaim 16 where the temperature of the preliminary reaction zone is at atemperature in the range from 200 to 380° F. in liquid phase.
 18. Theprocess according to claim 11 wherein the ammonium carbamates areremoved from the feeds by decomposition to ammonia and carbon dioxide.19. The process according to claim 18 wherein the temperature of thepreliminary reaction zone is at a temperature in the range from 200 to380° F. in liquid phase.
 20. The process according to claim 11 whereinthe said high boiling electron donor atom compound comprises triethyleneglycol dimethyl ether.